TestFlight is Currently Unavailable Message

Question

I am getting message \"TestFlight is Currently is Currently Unavailable. Try again later\" when trying to install from TestFlight app. My outside testers get a message \"[Ap

Answer 1

Problems with certificates have been identified in several answers as the cause to this message. However, there's another possible (yet stupid) cause that leads to this situation.

You may see this error if your device's date/time is not set correctly. So make sure you set the correct date or "Set Automatically" is enabled under Settings > Date & Time.

I spent 2 days "trying again later" until I realized this.